The performance and life of ferrofluid seals is strongly influenced by the operating temperature of the ferrofluid. This problem has been described by Raj et.al. in U.S. Pat. No. 4,357,021 issued 11/2/82.
"It is known that a temperature gradient across the ferrofluid O-ring seal is produced, as a result of the heat generated by the viscous shearing of the ferrofluid between the rotating spindle shaft and the inner diameter of the stationary pole pieces. Some of this heat is conducted away through the pole pieces and the spindle shaft. Thus, the operating ferrofluid temperature depends on the heat-sink capabilities of the seal materials and structure, which, in turn, determines the ferrofluid evaporation rate and, therefore, the life of the seal."
Raj et.al. in U.S. Pat. No. 4,357,021, Raj in U.S. Pat. No. 4,357,022 issued 11/2/82, Yamamura et.al. in U.S. Pat. No. 4,340,233 issued 7/20/82 and Yamamura in U.S. Pat. No. 4,357,023 issued 11/2/82 describe means whereby heat generated in rotating ferrofluid seals may be removed. The various schemes described in the above cited patents are summerized in U.S. Pat. No. 4,357,023, column 2, line 57 through column 3, line 42.
"It has been discovered that the life of a ferrofluid rotary-seal apparatus may be extended through the proper selection of the seal housing material and housing geometry, so as to conduct heat away from the ferrofluid in one or more of the gaps of the seal. The rapid removal of heat from the ferrofluid permits a lower ferrofluid temperature during shaft operation, resulting in reduced ferrofluid loss and an extention of seal life. An extended housing may be used, which extension overlaps the top of one or more pole pieces, or where the housing extends in contact with and along one side, preferably the outside, of the pole pieces heat-conductive, heat-exchange and contacting relationship. An extended housing, wherein the extension is a sheet material and is formed by swaging or staking along the outside of one or more of the pole pieces, has been found to extend seal life, as a result of the improved thermal path for heat conduction. In one embodiment, the housing extension may be formed integrally as part of the housing or formed separately adjacent the pole pieces by cladding or adhesive techniques. The conductive extension element may extend along side the pole pieces and toward and into a close relationship with the end of the pole piece forming the gap with the surface of the rotary shaft, to hold the sealing ferrofluid, so that a thermal path extends from the area of the gap alongside the pole piece to the top or body of the housing which serves as a heat sink. It has been found, particularly with extended housings on exclusion seals used with computer-disc-drive spindles with diester ferrofluid, that a temperature reduction of 5 C or more of the ferrofluid can be obtained, permitting an extension of seal life. PA1 In another embodiment, the heat-conductive extension can be placed on both pole pieces, to extend seal life for single-stage and multiple-stage ferrofluid seals. The extension on one pole piece extends the seal life of the ferrofluid at that particular gap, while the ferrofluid at the other gap, running at a higher temperature, is evaporated preferentially, to provide, after such stage failure or evaporation, an air gap. If desired, the housing material may be so designed to serve as a heat sink, or to designate or otherwise conduct the accumulated heat away to a heat sink of lower temperature, depending on the degree of seal life extension desired and the housing material and geometry used. PA1 The housing and extension should be composed of a nonmagnetic material which is highly thermal-conductive. Typical useful materials include metals, such as non-magnetic stainless steel (series 300), copper, aluminum and other metals. The removal of heat is particularly helpful in extending seal life, where the ferrofluid has a high viscosity and, thus, provides more heat due to shearing forces."
For heat to reach the heat conductive housing, it must travel through the magnetic pole pieces. In general, magnetic materials have poor thermal conductivity which when combined with a relatively long thermal path results in a large temperature differential between the ferrofluid seal and the heat conductive housing. Since ferrofluids should generally be operated at 50.degree. C. or less, a large temperature differential severely inhibits performance.
The use of heat conductive extension elements alongside the pole pieces is also proposed. This approach suffers from the shortcoming of being only suitable for one or two stage seals, i.e., external surfaces. If attempted for use in multi-stage seals, i.e., for vacuum use, the high thermal conductivity, nonmagnetic metal extension elements would interrupt the magnetic circuit, thereby compromising the seals effectiveness. Also, an undesirable temperature gradient is set up in radial direction.